The present invention relates to a magnetic bearing spindle for use in manufacturing processing machinery (for metal molding or aluminum processing such as scrolling), a turbo molecular pump of semiconductor equipment or the like.
The issues of a high-speed spindle for machining, taken as an example, will be described below.
In recent years, there has been a growing demand for high-speed cutting work in the field of machining. The high-speed cutting is expected to produce the effects of improving the production efficiency, improving the processing accuracy and prolonging the operating life of the tool by virtue of a reduction in cutting resistance, reducing the cost of the mold and the like, and reducing the processing time by virtue of cutting from an integrated material into a shape in a stroke.
The recent requirements for the product quality care about not only the quality of the processed surface (i.e., the shape accuracy and surface roughness) but also the defect beneath the processed surface and the presence or absence of a transubstantiated layer. Accordingly, there is a great expectation for high-speed cutting that receives little influence from the generation of heat accompanying the removal of metal and is able to reduce the cutting resistance.
For the spindle that decisively dominates the capabilities of the processing machinery, there has conventionally been mainly used a support structure with ball bearings. In response to the aforementioned demands for high-speed cutting, there have been developments for the increase in speed by improving the lubricating system and adopting a ceramics bearing or the like.
On the other hand, an active control type magnetic bearing spindle that supports a rotating body in a non- contact manner by magnetic levitation has attracted a great deal of attention as a spindle that has the possibility of exceeding the limit of the ball bearing system in recent years.
FIG. 15 illustrates an example of the magnetic bearing spindle, in which are shown a spindle main shaft 500, a motor rotor 501, and a motor stator 502. There are also shown front side radial bearings 503 and 504, rear side radial bearings 505 and 506, and thrust bearings 507 and 508. These combinations are each being constructed of the rotor located on the rotating side and the stator located on the stationary side. There are further shown radial displacement sensors 509 and 510 located on the front and rear sides, respectively, a thrust displacement sensor 511, protecting bearings 512 and 513, and a casing 514.
The fundamental capability of the processing use spindle is normally evaluated by the magnitude of a DN value (spindle diameter x number of revolutions).
In the case of the ball bearing spindle, which has undergone a variety of improvements in recent years, it is considered that the limitation of the DN value is practically about 2.5 million taking the operating life into account due to the accompaniment of mechanical sliding lubrication.
By contrast, in the case of the magnetic bearing, it is possible to provide a spindle of which the DN value surpasses the DN value of the ball bearing by making the best use of the features of non-contact rotation that can ensure semipermanent use. In order to satisfy the demands for the aforementioned high-speed high-rigidity structure on the processing side, there have been carried out trials for increasing the spindle main shaft diameter and rotating the same at higher speed. The large main shaft diameter is desired because the inertial rigidity (the dynamic effect in which the main shaft axial center tries to keep one direction) is greater in a high-speed operation and a blade tool of a larger diameter can be gripped because the main shaft diameter is greater.
However, it was discovered that the magnetic bearing, which was expected to have a small loss because of the non-contact structure, unexpectedly caused a great frictional loss as the result ova striving for a higher DN value. The principal factor of the above result is-the eddy current loss of the radial bearing.
FIGS. 16A and 16B show the principle of the radial bearing that has conventionally been used, in which are shown a rotor iron core 600 (corresponding to 503 of FIG. 15) constructed of electromagnetic steel plates, a stator iron core 601 (corresponding to 504 of FIG. 15), and a winding 602. In the figure, the flow of magnetic flux is indicated by the arrow 603. In the radial magnetic bearing, the rotor is retained at the center in a non-contacL manner by attracting the rotor 600 by magnetic forces in vertical and horizontal directions.
Because of the rotation, one key factor of the rotor iron core is that is successively faces magnetic poles 604 in the order of N.fwdarw.S.fwdarw.S.fwdarw.N (there is another case of N.fwdarw.S.fwdarw.N.fwdarw.S as described later) as shown in FIG. 16A, so that the direction and magnitude of the magnetic flux 603 varies. Consequently, a varying induction electromotive force is generated in the rotor iron core 600, so that an eddy current flows. In order to reduce this eddy current loss, the rotor iron core 600 normally has a laminate structure formed by stacking thin electromagnetic steel plates (silicon steel plates).
If the rotating section of the magnetic bearing is constructed for the achievement of a spindle having a high DN value (a large main shaft diameter and a greater number of revolutions), the following issues occur.
(1) If electromagnetic steel plates of a high resistivity, a small iron loss, and a small plate thickness are used in order to reduce the eddy current loss, there is a restriction on the permitted number of revolutions due to a limitation of the mechanical strength of the material with respect to a stress generated by a centrifugal force so long as the material is identical. The stress generated by the centrifugal force depends on the peripheral velocity of the rotating body, and this naturally leads to the limitation of the DN value.
(2) Conversely, when electromagnetic steel plates that endure a large number of revolutions and have a large plate thickness, a low resistivity, and a large iron loss are used, an abnormal temperature rise is caused on the main shaft by the heat generation due to a. large eddy current loss so long as the material is identical. This temperature rise exerts a great deal of bad influence on the reliability of the rotating main shaft constructed of composite components. The main shaft of the magnetic bearing is normally constructed of a motor, electromagnetic steel plates of the magnetic bearing, a ring for fastening the plates, a disk for the thrust bearing, a tooling member provided by utilizing the inside of the main shaft, and so on. The main shaft subjected to severe conditions at high speed and high temperature causes the troubles of destruction, deformation, and the like of these composite components.
(3) The loss can be reduced by reducing the bias current to be formed through the electromagnet of the radial bearing, and reducing the face width or the axial length of the electromagnet, or by taking similar measures. However, the rigidity and loading capability are concurrently reduced, and therefore, the DN value is hard to increase.